1. Field of the Invention
The present invention relates to protection techniques for semiconductor devices and, more particularly, is directed towards a novel terminal protection device for monolithic mircowave integrated circuits that prevents damage to monolithic microwave integrated circuits upon the introduction of extreme RF energy.
2. Description of the Prior Art
The trend in advanced electronic systems is toward increasing integration, reliability, radiation hardness, and lower cost in large volume production. For example, monolithic microwave integrated circuits (MMICs) have been developed to replace standard microwave integrated circuits with the benefits of reduction in size and cost while also improving reliability. Unfortunately, these MMICs are inherently susceptible to damage from extraneous RF energy. High levels of energy injected into the MMIC at the input terminal can cause circuit elements on the semiconductor substrate to be damaged. Consequently, some sort of terminal protection device (TPD) and technique is required to eliminate this problem. These TPDs must not only eliminate the possibility of RF damage, but must also be consistent with the MMIC goals of low cost, small size, high level of integration and high reliability.
While it is well known how to protect semiconductor devices from over voltage conditions, these prior art devices and techniques are not suitable in MMICs. Traditional TPDs are discrete devices which are inserted into the RF transmission path at some convenient point such as between the antenna and radar transciever. However, MMICs will be connected, in many cases, to the antenna directly without intervening RF transmission paths suitable for traditional TPD insertion. Other standard solid state protection techniques such as PIN limiters are unable to handle the extreme powers and, in addition, require additional solid state processing steps which increases MMIC costs. Other techniques such as that disclosed in U.S. Patent 2,793,331 to Lamb are inconvenient, are incompatible with the high level of integration of MMICs and are slow in triggering. Gas discharge devices are slow in responding to over voltages in that the formation of the discharge plasma which carries the current must be initiated by a free electron. The electron needed to initiate the discharge is termed a "seed electron." An electron may be freed from one of the surfaces by field emission, but extremely high voltages are required. An electron may be created by an occasional cosmic ray, but this happens too infrequently to provide a small turn on delay, although this is the normal mode of providing seed electrons. The most popular method of providing seed elelctrons for T-R tubes, for example, is to place a radioactive source close to the volume in which the arc is to be formed. The radioactive source provides a constant supply of seed electrons and the only delay is the formation time of the arc. A radioactive source may, however, degrade the performance of the semiconductor junctions in the MMIC package.
Consequently, it is desirable to have a TPD in which the TPD is integral to the MMIC and fast acting. It is also desirable to have a simple means to protect these MMIC devices from extremes RF energy which is not inconsistent with the MMIC goals of low cost, small size, high level of integration and high reliability.